The present application relates generally to an improved data processing apparatus and method and more specifically to mechanisms for simultaneous optical proximity correction and decomposition for double exposure lithography.
Optical lithography is a crucial step in semiconductor manufacturing. The basic principle of optical lithography is quite similar to that of chemistry-based photography. The images of the patterned photo-mask are projected through the high-precision optical system onto the wafer surface, which is coated with a layer of light-sensitive chemical compound, e.g. photo-resist. The patterns are then formed on the wafer surface after complex chemical reactions and follow-on manufacturing steps, such as development, post-exposure bake, and wet or dry etching.
The resolution of the photo-lithography system (R) can be described by the well-known Rayleigh's equation:
  R  =                    k        1            ⁢      λ        NA  in which λ is the wavelength of the light source, NA is the numerical aperture, and k1 is the factor describing the complexity of resolution enhancement techniques. As the very-large-scale integration (VLSI) technology pushes further into nanometer region, the feasible wavelength of the photo-lithographic system remains unchanged at 193 nm. Although there is anticipation that extreme ultraviolet lithography (EUVL) with the wavelength of 13 nm will replace traditional optical lithography, the availability of EUVL remains uncertain due to technical challenges and cost issues. On the other hand, the physical limit of dry lithography of NA is 1.0. The recently introduced immersion lithography has bigger NA (1.2), but it is harder to further increase NA to even higher values. Thus it is commonly recognized that k1 remains a cost effective knob to achieve finer resolution.
Due to the unavoidable diffraction, the optical lithography system is lossy in the sense that only low frequency components of the electromagnetic field can pass the optical system. Given a target layout of shapes that are desired to be manufactured, masks are generated that account for the non-linearities introduced by the lithographic process that prints wafer features that resemble the target. As the gap between the required feature size and lithography wavelength gets bigger, the final wafer images are quite different from the patterns on the mask. In the past few years, resolution enhancement techniques (RETs) have become necessary in order to achieve the required pattern density. One well-known RET is the optical proximity correction (OPC), in which the mask patterns are intentionally “distorted” so that the desired image can be formed on the wafer. Other commonly used RETs are sub-wavelength resolution assist features (SRAF) and phase-shift masks (PSM). Nowadays, considerable amount of computing power has to be dedicated to these post-layout processes (often referred as data prep).
Optical proximity correction OPC involves simulating the wafer image given a mask. OPC extracts the geometric error between the simulated wafer feature and the target. The geometric error is called edge placement error (EPE). A cost function is defined as the summation of the EPEs across a layout and modifications of the mask are performed so as to minimize this cost function. OPC arrives at a final mask solution after several iterations of image simulations and mask modifications. One drawback of this type of mask modification is that the image simulation is performed at a single process point, most often under nominal process conditions. Hence, OPC cannot compensate for any variations that may occur during the lithographic process such as variations in lithographic dose and focus.
As we further scale into the deep submicron domain using the same lithographic technology, the process of double patterning has been developed as a cost-effective way to overcame scaling challenges. One popular form of double patterning is double exposure lithography, where a given layout is split or decomposed into two sets of patterns, each of which is printed using a separately optimized illumination. One example of double exposure is double dipole lithography (DDL). This method works on the principle that a horizontal dipole is optimal for printing vertical features, while a vertical dipole is optimal for printing horizontal features. It then decomposes the layout into two patterns—one containing primarily horizontal patterns, and the other containing primarily vertical patterns each of which is printed with its corresponding dipole. However, the decomposition process itself is not trivial as there exist different types of two-dimensional shapes that cannot always be classified as either horizontal or vertical. Some examples of such patterns are jogs and line-ends.